subsurface/device.c

#include <string.h>
#include "dive.h"
#include "device.h"

/*
 * Good fake dive profiles are hard.
 *
 * "depthtime" is the integral of the dive depth over
 * time ("area" of the dive profile). We want that
 * area to match the average depth (avg_d*max_t).
 *
 * To do that, we generate a 6-point profile:
 *
 *  (0, 0)
 *  (t0, max_d)
 *  (t1, max_d)
 *  (t2, d)
 *  (t3, d)
 *  (max_t, 0)
 *
 * with the same ascent/descent rates between the
 * different depths.
 *
 * NOTE: avg_d, max_d and max_t are given constants.
 * The rest we can/should play around with to get a
 * good-looking profile.
 *
 * That six-point profile gives a total area of:
 *
 *   (max_d*max_t) - (max_d*t1) - (max_d-d)*(t4-t3)
 *
 * And the "same ascent/descent rates" requirement
 * gives us (time per depth must be same):
 *
 *   t1 / max_d = (t3-t2) / (max_d-d)
 *   t1 / max_d = (max_t-t4) / d
 *
 * We also obviously require:
 *
 *   0 <= t1 <= t2 <= t3 <= t4 <= max_t
 *
 * Let us call 'd_frac = d / max_d', and we get:
 *
 * Total area must match average depth-time:
 *
 *   (max_d*max_t) - (max_d*t1) - (max_d-d)*(t4-t3) = avg_d*max_t
 *      max_d*(max_t-t1-(1-d_frac)*(t4-t3)) = avg_d*max_t
 *             max_t-t1-(1-d_frac)*(t4-t3) = avg_d*max_t/max_d
 *                   t1+(1-d_frac)*(t4-t3) = max_t*(1-avg_d/max_d)
 *
 * and descent slope must match ascent slopes:
 *
 *   t1 / max_d = (t3-t2) / (max_d*(1-d_frac))
 *           t1 = (t3-t2)/(1-d_frac)
 *
 * and
 *
 *   t1 / max_d = (max_t-t4) / (max_d*d_frac)
 *           t1 = (max_t-t4)/d_frac
 *
 * In general, we have more free variables than we have constraints,
 * but we can aim for certain basics, like a good ascent slope.
 */
static int fill_samples(struct sample *s, int max_d, int avg_d, int max_t, double slope, double d_frac)
{
	double t_frac = max_t*(1-avg_d/(double)max_d);
	int t1 = max_d / slope;
	int t4 = max_t - t1*d_frac;
	int t3 = t4-(t_frac-t1)/(1-d_frac);
	int t2 = t3-t1*(1-d_frac);

	if (t1 < 0 || t1 > t2 || t2 > t3 || t3 > t4 || t4 > max_t)
		return 0;

	s[1].time.seconds = t1; s[1].depth.mm = max_d;
	s[2].time.seconds = t2; s[2].depth.mm = max_d;
	s[3].time.seconds = t3; s[3].depth.mm = max_d * d_frac;
	s[4].time.seconds = t4; s[4].depth.mm = max_d * d_frac;

	return 1;
}

struct divecomputer* fake_dc(struct divecomputer* dc)
{
	static struct sample fake[6];
	static struct divecomputer fakedc;

	fakedc = (*dc);
	fakedc.sample = fake;
	fakedc.samples = 6;

	/* The dive has no samples, so create a few fake ones */
	int max_t = dc->duration.seconds;
	int max_d = dc->maxdepth.mm;
	int avg_d = dc->meandepth.mm;

	memset(fake, 0, sizeof(fake));
	fake[5].time.seconds = max_t;
	if (!max_t || !max_d)
		return &fakedc;

	/*
	 * We want to fake the profile so that the average
	 * depth ends up correct. However, in the absense of
	 * a reasonable average, let's just make something
	 * up. Note that 'avg_d == max_d' is _not_ a reasonable
	 * average.
	 */
	if (avg_d < max_d / 10 || avg_d >= max_d) {
		avg_d = (max_d+10000)/3;
		if (avg_d > max_d)
			avg_d = max_d * 2 / 3;
	}
	if (!avg_d)
		avg_d = 1;

	/*
	 * Ok, first we try a basic profile with a specific ascent
	 * rate (5 meters per minute) and d_frac (1/3).
	 */
	if (fill_samples(fake, max_d, avg_d, max_t, 5000.0 / 60, 0.33))
		return &fakedc;

	/*
	 * Ok, assume that didn't work because we cannot make the
	 * average come out right because it was a quick deep dive
	 * followed by a much shallower region
	 */
	if (fill_samples(fake, max_d, avg_d, max_t, 10000.0 / 60, 0.10))
		return &fakedc;

	/*
	 * Uhhuh. That didn't work. We'd need to find a good combination that
	 * satisfies our constraints. Currently, we don't, we just give insane
	 * slopes.
	 */
	if (fill_samples(fake, max_d, avg_d, max_t, 10000.0, 0.01))
		return &fakedc;

	/* Even that didn't work? Give up, there's something wrong */
	return &fakedc;
}